1. Field of the Invention
The present invention generally relates to a memory circuit, and particularly relates to a nonvolatile memory circuit which is capable of retaining stored data in the absence of a power supply voltage.
2. Description of the Related Art
Nonvolatile semiconductor memory devices, which can retain stored data even when power is turned off, include flash EEPROMs employing a floating gate structure, FeRAMs employing a ferroelectric film, MRAMs employing a ferromagnetic film, etc.
In the case of EEPROMs, there is a need to manufacture a transistor having a special structure comprised of a floating gate. In the case of FeRAMs and MRAMs, which achieve nonvolatile storage by use of a ferroelectric material and a ferromagnetic material, respectively, there is a need to form and process a film made of these respective materials. The need for such transistor having a special structure and the need for such film made of a special material are one of the factors that result in an increase in the manufacturing costs.
PCT/JP2003/016143, which was filed on Dec. 17, 2003, the entire contents of which are hereby incorporated by reference, discloses a nonvolatile memory cell (i.e., a basic unit of data storage) comprised of a pair of MIS (metal-insulating film-semiconductor) transistors that have the same structure as ordinary MIS transistors used for conventional transistor functions (e.g., switching function). Namely, these memory cell transistors use neither a special structure such as a floating gate nor a special material such as a ferroelectric material or a ferromagnetic material. These MIS transistors are configured to experience an irreversible hot-carrier effect on purpose for storage of one-bit data. A difference in the transistor characteristics caused by the hot-carrier effect represents one-bit data “0” or “1”.
Specifically, when one of the two transistors is subjected to a hot-carrier effect, a difference in the ON current develops between the two transistors. The difference in the ON current may be detected by a one-bit static memory circuit (latch) coupled to the transistor pair. Such latch circuit and the pair of memory cell transistors together constitute a memory cell circuit, i.e., a basic unit of data storage.
The pair of memory cell transistors serving as nonvolatile data storage is not required to produce a large drain current, but are required to generate an irreversible hot-carrier effect in a short time with a low voltage stress applied thereto. Other MIS transistors used in the memory cell circuit, on the other hand, are required to allow a large drain current to flow, and are also required to be less susceptible to the hot carrier effect. In this manner, conflicting characteristics are required of the MIS transistors used in a single memory cell circuit.
Accordingly, there is a need for a memory cell circuit and nonvolatile semiconductor memory device that can satisfy the conflicting requirements for transistor characteristics between nonvolatile memory cell transistors and other transistors used in the memory cell circuit utilizing the hot carrier effect for nonvolatile data storage.